The technique of coextrusion of at least two thermoplastic materials is well known and it is described, in particular, in POLYMER PLASTICS TECHNOLOGY AND ENGINEERING, Volume 3, pages 40-68: "Coextruded Films--Process and Properties" by John E. Guillotte.
Generally speaking, three methods of proceeding with the coextrusion of thermoplastic materials are known using conventional extruders whose number is equal to the number of polymers to be extruded. The first method consists of extruding the polymers separately and combining them as they exit the extrusion dies. The second method consists of feeding a single die by means of at least two extruders, with the die including as many passages as there are extruders and, consequently, polymers to be extruded. The streams of polymers meet at the level of the exit orifice of the die, thus perceptibly just prior to exiting from the latter. The third method consists of feeding a stream distributor by means of the desired number of extruders. In this distributor, the polymers combine into a single stream which is supplied to the die. In these processes, the respective flow rates of the extruders usually make it possible to control the relative thicknesses of the extruded polymers.
While numerous polymers can be thus coextruded, it has not been possible to satisfactorily combine a polyvinylidene fluoride (hereinafter "PVF.sub.2 ") with other polymers by these techniques. The reason, it is believed, is probably due to the lack of compatibility of the PVF.sub.2 with the other polymers and to the well-known lack of adhesion between fluorine-containing resins and the majority of thermoplastic polymers. This difficulty of associating (combining) PVF.sub.2 with the other thermoplastics is encountered even when the conventional laminating or veneering technique is applied to it. This consists in attempting to cause two films having been manufactured beforehand; one made of PVF.sub.2 and the other one made another thermoplastic resin, to adhere to each other under pressure in the heated state. Such attempts have even been carried out unsuccessfully by using a prefabricated PVF.sub.2 film and films of polyvinyl chloride, polystyrene, polymethyl methacrylate, or acrylonitrile-butadiene-styrene copolymer coming out of the extruder; that is to say in a practically molten state. Even under these conditions, after cooling, the two polymer layers can easily be separated.
In the present state of this art, when it is desired to combine a PVF.sub.2 resin with an "incompatible" thermoplastic resin (one that will not adhere to the PVF.sub.2) it is necessary to resort to an adhesive. This method has the drawback of requiring three operating phases:
1. preparation of the PVF.sub.2 film;
2. preparation of the thermoplastic film; and
3. application of the adhesive and pressing one film to the other.
These operations are not commercially practical in that they are slow and generally necessitate the use of adhesives having a solvent base, with these solvents being difficult to eliminate, and not permitting the immediate use of the composite material because of the time required for drying of the adhesive. Moreover, the laminate obtained lacks cohesion, with the interfaces remaining sensitive to all phenomena apt to cause ungluing. As a result, it can be said that by adhesion one does not obtain a unitary laminate, but a simple juxtaposition of thermoplastic elements whose final structure is heterogeneous.